Demand flow for air cooled chillers

ABSTRACT

In an air cooled chiller, a refrigerant pump and bypass valve connected in parallel feed refrigerant from the condenser to a receiver; the pump is activated in response to condenser pressure; and the bypass is used otherwise. The condenser fan is controlled based on power consumption by the air cooled chiller that varies with ambient conditions. An evaporator set point is reset to meet load conditions. An expansion valve for the evaporator is controlled based on chilled water temperature, Delta T, or information from an air handling unit. Feedback of valve setting or position, temperature of air, valve size, and/or importance of an air handling unit is used to control the flow of chilled water. A variable pressure curve or other relationship is used with the feedback to control flow of chilled water based on the load. Refrigerant temperature is controlled based on information from the air handling unit.

CROSS-REFERENCE TO OTHER APPLICATION

This application is a divisional of U.S. Non-Provisional patentapplication Ser. No. 14/460,190, filed Aug. 14, 2014, which is herebyincorporated by reference.

TECHNICAL FIELD

The invention relates generally to chilled water comfort cooling andindustrial process cooling systems and, in particular, chilled watercooling systems using an air cooled chiller.

BACKGROUND

Many buildings and campuses are cooled by chilled water plants. Ingeneral, these chilled water plants use a chiller to produce chilledwater, which is pumped to air handlers to cool building air. Chillers,air handlers, and other components of a chilled water plant are designedto operate at a specific chilled water entering and leaving temperature,or the difference, Delta T. At design Delta T, these components are attheir most efficient and can produce cooling output at their ratedcapacity. Low Delta T, which occurs when the entering and leavingtemperature become closer than the design Delta T, reduces efficiencyand cooling capacity of the chilled water plant and causes the chilledwater plant to use more energy than required for a given demand.

Many mitigation strategies have been developed to address Low Delta T,such as through the use of sophisticated sequencing programs andequipment ON/OFF selection algorithms. For example, chilled water plantsusing an air cooled chiller may cycle a condenser fan on and off orprovided variable condenser fan speeds. As another example, variablechilled water flow is used. In most instances, the chilled water plantoperator simply pumps more water to system air handlers to increasetheir output, but this has the compounding effect of further reducingthe already low Delta T. Also, increased pumping results in higher thannecessary pumping energy usage. These approaches may not completelyresolve the Low Delta T.

Air cooled chillers compound the inefficiency. Air cooled chillers areinefficient as compared to water cooled chillers. The condenser sectionin an air cooled chiller stores a low volume of liquid refrigerant,making low ambient temperature operation difficult because therefrigerant tends to build up in the condenser (i.e., stacking). Thisbuild-up causes system pressure to drop, and the chiller to shut down.The condenser fan section is generally set to maintain a constantcondensing temperature. This may be ideal for a small range of ambienttemperatures, but is usually too high or low at most times of the year.Nuisance trips may be caused by maintenance of the temperature.

SUMMARY

An air cooled chiller and the use of the air cooled chiller in a chilledwater plant are provided. For more efficient operation, a refrigerantpump and bypass valve connected in parallel feed refrigerant from thecondenser to a receiver. The pump is activated in response, at least inpart, to pressure in the condenser, and the bypass is used otherwise.The use of the refrigerant pump and receiver may avoid stacking,handling varying load conditions without forcing refrigerant through theevaporator.

Further efficiency may be provided by controlling the condenser fanbased on power consumption by the air cooled chiller. Power usage varieswith ambient conditions (e.g., air temperature, sunlight, and wind).Using power consumption feedback to control the fan may tune thecondensing based on ambient conditions without requiring various ambientcondition sensors.

Yet another improvement in efficiency is provided by resetting a setpoint of the evaporator to meet load conditions. An expansion valve forthe evaporator is controlled based on chilled water temperature, such asDelta T, and/or load information from an air handling unit. Resettingthe set point for refrigerant at the evaporator may assist in properrefrigerant flow relative to the demand.

In yet another improvement in efficiency, feedback of valve setting orposition, temperature of air, valve size, and/or importance of an airhandling unit is used to control the flow of chilled water. A variablepressure curve or other relationship is used with the feedback tocontrol flow of chilled water based on the load, resulting in bettercontrol of pressure or pressure differential. In addition to or as analternative to control of the chilled water flow, the refrigeranttemperature may be controlled based on information from the air handlingunit.

Any of the improvements may be used alone or in any combination.

In a first aspect, an air cooled chiller is provided. A refrigerant loophas a compressor fluidly connected to a condenser, the condenser fluidlyconnected to an evaporator, and the evaporator fluidly connected to thecompressor. A refrigerant pump and isolation valve are in therefrigerant loop and fluidly connect in parallel with and between thecondenser and the evaporator. A receiver fluidly connected with therefrigerant pump.

In a second aspect, a method is provided for controlling an air cooledchiller. Refrigerant is condensed in a condenser and evaporated in anevaporator. Pressure of the refrigerant is controlled, in part, with acompressor. The refrigerant is pumped in response to sensing pressurebetween the condenser and the evaporator.

In a third aspect, a system is provided for controlling air flow over acondenser. A fan is arranged to cause flow of air over the condenser. Ameter is configured to determine energy used by an air cooled chiller,which includes the condenser. A controller is configured to control airflow caused by the fan as a function of the energy.

In a fourth aspect, a method is provided for use of a chiller. Anaperture setting of a valve in an air handling unit is determined. Oneor more parameters from the group of: a discharge temperature of chilledwater at the air handling unit, a size of the valve of the air handlingunit, and an importance of the air handling unit are determined. Flow ofthe chilled water of the chiller is controlled as a function of theaperture setting and the one or more parameters.

In a fifth aspect, a method is provided for use of a chiller. Anaperture setting of a valve in an air handling unit is determined. Arefrigerant temperature in the chiller is controlled as a function ofthe aperture setting.

Other systems, methods, and/or features of the present embodiments willbecome apparent to one with skill in the art upon examination of thefollowing figures and detailed description. It is intended that all suchadditional systems, methods, features and advantages be included withinthis description, be within the scope of the invention, and be protectedby the accompanying claims. Additional features of the disclosedembodiments are described in, and will be apparent from, the followingDetailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

The components in the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.In the figures, like reference numerals designate corresponding partsthroughout the different views.

FIG. 1 is a block diagram illustrating an exemplary decoupled chilledwater plant;

FIG. 2 illustrates one embodiment of an air cooled chiller;

FIG. 3 is a block diagram illustrating a control arrangement;

FIG. 4 is a flow diagram illustrating an exemplary controller inoperation;

FIG. 5 is a flow diagram of one embodiment of a method for control of arefrigerant pump in an air cooled chiller;

FIG. 6 is a flow diagram of one embodiment of a method for control of acondenser fan of an air cooled chiller;

FIG. 7 is a flow diagram of one embodiment of a method for control ofchilled water flow;

FIG. 8 is a block diagram illustrating an exemplary chilled water plantoperable according to the Demand Flow variable pressure curve logicprinciples;

FIG. 9 is a flow diagram illustrating an exemplary Demand Flow condenserroutine or algorithm in operation;

FIG. 10 is a flow diagram of one embodiment of a method for control ofrefrigerant temperature;

FIG. 11 is a flow diagram illustrating an exemplary Demand Flowevaporator routine or algorithm in operation;

FIG. 12 is a flow diagram illustrating an exemplary routine or algorithmfor determining the P Exponent variable for a given pump;

FIG. 13 is a block diagram illustrating the exemplary chilled waterplant shown in FIG. 8 operating according to the disclosed Demand Flowvariable pressure curve logic principles disclosed herein;

FIG. 14 is a flow diagram illustrating an exemplary routine or algorithmfor control of a condenser pressure setpoint;

FIG. 15 is a flow diagram illustrating an exemplary routine or algorithmfor calculating air handling unit cooling demand;

FIG. 16 is a flow diagram illustrating an exemplary routine or algorithmfor fault detection in an air handling unit valve;

FIG. 17 is a flow diagram illustrating an exemplary routine or algorithmfor chilled water flow pump control;

FIG. 18 is a flow diagram illustrating an exemplary routine or algorithmfor chiller load calculation; and

FIG. 19 is a flow diagram illustrating an exemplary routine or algorithmfor resetting the chilled water temperature setpoint.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth inorder to provide a more thorough description of the present invention.It will be apparent, however, to one skilled in the art, that thepresent embodiments may be practiced without these specific details. Inother instances, well-known features have not been described in detailso as not to obscure the invention.

An air cooled chiller is created or modified to accept demand flowprocesses and integration of variable air volume (VAV) or other loadside management routines. For example, air cooled chillers arecontrolled as part of a chilled water plant, such as with the controldisclosed in U.S. Published Patent Application No. 2011/0301766, thedisclosure of which is incorporated herein by reference. In oneembodiment, the control of the chilled water disclosed therein isperformed. Some or any of the controls may be used, such as using thedemand flow, variable pressure curve logic, condenser pressure set pointor reset, evaporator pressure set point or reset, chilled watertemperature set point or reset, and/or other control for the chilledwater and/or the chiller with the air cooled chiller disclosed herein.For example, the chilled water flow is controlled with the variablepressure curve logic with feedback from the load (e.g., air handlingunit) in the air cooled chiller. As another example, the expansion valveof the evaporator in the air cooled chiller is controlled to reset therefrigerant evaporator set point to meet load conditions (e.g., byvarying the chilled water set point to meet demand).

In one embodiment, air cooled chiller operations are integrated withload information from space and ambient conditions to produce the lowestannual kW/ton. The space information may be feedback from one or moreair handling units, such as feedback of valve position (amount open orclosed), temperature, valve size, and/or importance of the air handlingunit to the overall chilled water plant. The ambient conditions (e.g.,temperature, sun light, and/or wind) are accounted for by measuring thepower used by the air cooled chiller rather than complications fromdownloading weather information and/or sensing weather information.

In another embodiment, a floating refrigerant chiller set point of theevaporator is tied to load. Where the chilled water flow rate is setbased on demand, the flow rate may reach a minimum. At the minimum, therefrigerant set point may be increased. The refrigerant set point may bevaried based on the load rather than waiting until a minimum is reached,such as controlling expansion valves of a evaporator based oninformation from the air handling unit.

In another embodiment, floating head pressure is tied to the chilleroutput. The refrigerant pressure in the evaporator and/or condenservaries. This pressure forces the refrigerant through the expansion valveof the evaporator. By controlling the pressure at one or more locationsbased on chiller output, more efficient operation may be provided.

In yet another embodiment, variable pressure control logic is used todrive chiller set points and output. Alternatively or additionally,chilled water flow and/or refrigerant temperature is controlled based ondemand information from air handling units. The information may be avalve position, valve size, temperature of air, and/or importance of theair handling unit. The pressure in the air cooled chiller is controlledbased on load, as indicated by the demand information. The chilled waterflow rate affects the settings of the evaporator, which in turn altersthe pressure of the refrigerant elsewhere in the air cooled chiller.

Different approaches may provide different amounts of increases inefficiency. For example, using the various improvements discussed above,air cooled chiller seasonal efficiency may improve by approximately 40%.

Referring to U.S. Published Patent Application No. 2011/0301766, thecontrol of chilled water is provided in a chilled water system using anair cooled chiller. A critical zone reset may increase cooling output.For example, a flow rate may be increased by resetting a Delta T when awater valve of the air handler unit opens beyond a particular threshold.This increase to the flow rate causes an increase to cooling output atthe air handler. In addition to valve opening, the valve size, airhandler importance, and/or temperature (e.g., temperature of air leavingthe air handling unit) are used to control the timing and/or magnitudeof the reset.

Demand flow, in other exemplary embodiments, may further be implementedaccording to demand flow variable pressure curve logic (VPCL). Demandflow VPCL represents an operational strategy that optimizes the totalenergy consumption of the chiller, chilled water pump, the condenserfan, and air handling unit along a calculated efficiency curve. Theoperational strategy represented by demand flow VPCL individually andholistically optimizes the active pressure curve setpoints for each ofthe pumps operable within a chilled water plant. In response to theoptimization of the active pressure curve setpoints for each of thepumps, the speed and energy usage associated with the condenser fan andthe air handling unit may also be adjusted and optimized. The settingsof valves, such as the evaporator expansion valves, may be adjusted.

The air cooled chiller is implemented in a chilled water systemoperating pursuant to Demand Flow. Demand Flow, as described herein,refers to methods and apparatus to reduce or eliminate Low Delta T andto improve chilled water plant efficiency. Demand Flow may beimplemented in retrofit projects for existing chilled water plants aswell as new installations or designs of chilled water plants. Chilledwater plant refers to cooling systems utilizing chilled water to providecomfort cooling or chilled water for some process need. Such chilledwater plants are typically, but not always, used to cool campuses,industrial complexes, commercial buildings, and the like.

In general and as will be described further below, Demand Flow utilizesvariable flow or pumping of chilled water within a chilled water plantto address Low Delta T and to substantially increase the efficiency of achilled water plant. Variable flow under Demand Flow maintains a Delta Tfor chilled water plant components where the Delta T is at or near thedesign Delta T for the components. As a result, Demand Flowsubstantially increases the operating efficiency of chilled water plantsand components thereof, resulting in savings in energy costs. Theincreased efficiency provided by Demand Flow may also reduce pollution.Furthermore, Demand Flow may also increase the life expectancy ofchilled water plant components by operating these components near or attheir specified entering and leaving chilled water temperatures, ordesign Delta T, unlike traditional variable or other pumping techniques.

Demand Flow provides increased efficiency regardless of cooling demandor load by operating chilled water plant components in a synchronousfashion. In one or more embodiments, this occurs by controlling chilledwater pumps and a condenser fan to maintain a Delta T at particularcomponents or points of a chilled water plant. In general, Demand Flowoperates on individual condenser fans or water pumps to maintain a DeltaT across a particular component or point of a chilled water plant. Forexample, primary chilled water pumps may be operated to maintain a DeltaT across a chiller, secondary chilled water pumps may be operated tomaintain a Delta T across plant air handlers, and condenser fans may beoperated to maintain a Delta T across a condenser.

The control of individual pumps or motors (and flow rate) in this mannerresults in synchronized operation of a chilled water plant. Thissynchronized operation balances flow rates in the chilled water plant,which significantly reduces or eliminates Low Delta T Syndrome andrelated inefficiencies.

Because flow rates are controlled by Demand Flow to maintain a Delta Tand not to maintain Delta P or a particular cooling output at plant airhandlers, there may be situations where the flow rate is too low toproduce the desired amount of cooling output in certain areas based onsystem diversity. To address this, Demand Flow includes a featurereferred to herein as a critical zone reset, which allows the Delta Tmaintained by Demand Flow to be reset to another, typically lower, valuebased on a specific need of the system that is not being fully met atthe required flow rate of the system. This can be due to inadequatepiping, incorrectly sized air handlers for the load being served, or anynumber of unforeseen system anomalies. This allows additional cooling tobe provided by maintaining a new or reset Delta T generally byincreasing chilled water flow. The application of Demand Flow has asynergistic effect on air handlers as well as chillers, pumps, and othercomponents of a chilled water plant. This may result in reduced netenergy usage while maintaining or even increasing the rated capacity forthe chilled water plant. Under Demand Flow, little or no excess energyis used to provide a given level of cooling.

FIG. 1 shows an exemplary decoupled chilled water plant. In otherembodiments, a single loop is provided in a direct-primary chilled waterplant. Other types of chilled water plants may be used.

In addition to describing the chilled water plant, Low Delta T isdescribed with regard to FIG. 1. As shown, the chilled water plantincludes a primary loop 104 and a secondary loop 108. Each loop 104,108may have its own entering and leaving water temperature, or Delta T.During operation of a decoupled chilled water plant, chilled water isproduced in a production or primary loop 104 by one or more chillers112. This chilled water may be circulated in the primary loop 104 by oneor more primary chilled water pumps 116. Chilled water from the primaryloop 104 may then be distributed to a building (or other structure) by adistribution or secondary loop 108 in fluid communication with theprimary loop 104. Within the secondary loop 108, chilled water may becirculated by one or more secondary chilled water pumps 120 to one ormore air handler units 124. The air handler units 124 allow heat fromthe building's air to be transferred to the chilled water, such asthrough one or more heat exchangers. This provides cooled air to thebuilding. Typically, building air is forced or blown through a heatexchanger in an air handler unit 124 to better cool a volume of air. Thechilled water leaves the air handler units 124 returning to thesecondary loop 108 at a higher temperature due to the heat the chilledwater has absorbed via the air handler unit or units 124.

The chilled water then leaves the secondary loop 108 and returns to theprimary loop 104 at the higher temperature. Both the primary loop 104and secondary loop 108 (as well as the chilled water plant componentsattached to these loops) have an entering water temperature and aleaving water temperature, or Delta T. In an ideal situation, theentering and leaving temperatures for both loops would be at theirrespective design Delta Ts. Unfortunately, in practice, the chilledwater loops operate at chronic low Delta T.

Low Delta T occurs for a variety of reasons. In some cases, low Delta Toccurs because of an imperfect design of the chilled water plant. Thisis relatively common due to the complexity of chilled water plants anddifficulty in achieving a perfect design. To illustrate, air handlerunits 124 of the secondary loop 108 may not have been properly selectedand thus chilled water does not absorb as much heat as expected. In thiscase, the chilled water from the secondary loop 108 enters the primaryloop 104 at a cooler temperature than expected, resulting in low DeltaT. It is noted that, due to imperfect design and/or operation, a chilledwater plant may be operating at low Delta T under various loads,including design condition loads.

Low Delta T also occurs as cooling output is lowered to meet a load thatis less than the design condition. As output is lowered, chilled waterflow, chilled water Delta T, and other factors become unpredictable,often resulting in low Delta T. In fact, in practice, Delta P flowcontrol schemes invariably result in low Delta T at some, if not all,chilled water plant components. For example, to reduce cooling outputfrom design conditions, one or more chilled water valves of the chilledwater plant's air handler units 124 may be closed (partially orcompletely). This reduces chilled water flow through the air handlerunits 124 and thus less cool air is provided. However, now that thechilled water valves are partially closed, the chilled water absorbsless heat from the air as the water flows through the air handler units124 at a higher rate than necessary. Thus, the chilled water leaving theair handler units 124 is not as “warm” as intended in the design. As aresult, the chilled water leaving the secondary loop 108 for the primaryloop 104 is cooler than desired, causing low Delta T in both loops.

Another cause of low Delta T is bypass mixing caused by excess flowwithin the primary loop 104, the secondary loop 108, or both. Toillustrate, 300 gallons per minute (GPM) excess flow of 54 degree waterfrom the secondary loop 108 would mix with 40 degree chilled water fromthe primary loop 104 in the bypass 128, raising the temperature of thesecondary loop's chilled water to 42 degrees. Now, the secondary loop'schilled water has a temperature higher than the primary loop's chilledwater. This causes low Delta Tin the primary loop 104 and the secondaryloop 108 and a corresponding reduction in cooling capacity.

Bypass mixing of chilled water streams is also undesirable because itexacerbates low Delta T. Excess flow and bypass mixing also cause excessenergy usage for a given cooling demand.

In many cases, these measures (e.g. increased chilled water pumping,opening of air handler water valves, increased air supply air movement)do not fully compensate for the artificial reduction in cooling capacitycaused by low Delta T. Thus, the chilled water plant is simply unable tomeet the demand for cooling even though this level of demand may bebelow its rated chilling capacity.

Low Delta T also occurs in direct-primary chilled water plantconfigurations (i.e. non-decoupled chilled water plants), even thoughsuch configurations generally do not have the problem of mixing buildingreturn water with production supply water. Direct-primary systemsinvariably have a plant or system bypass, 3-way valves, or both in orderto maintain minimum flow through the system.

FIG. 2 illustrates an exemplary air cooled chiller 112 for use in achilled water plant. The air cooled chiller 112 includes a refrigerantloop 200. The refrigerant loop 200 includes a compressor 202, condenser204, refrigerant pump 208, by pass valve 210, receiver 212, andevaporator 206 with expansion valves 214 and solenoid valves 216. Acondenser fan 218 for forcing air over the condenser 204 is alsoprovided in the air cooled chiller 112. For illustrative purposes, thechilled water pipes 220 are shown connected with the air cooled chiller112. Additional, different or fewer components may be provided, such asfurther valves, pumps, and/or sensors. In one example, a controller 302is provided. The arrow to and from the controller 302 represents inputsfrom various sensors and outputs to various components for controloperation.

The components in the refrigerant loop 200 fluidly connect with eachother. Pipes, lines, or tubes housing refrigerant as a vapor or gasand/or as a liquid connect between the components. The connection may bedirect, such as from the evaporator 206 to the compressor 202 or thecompressor 202 to the condenser 204. The connection may be indirect,such as there being one or more intervening components. For example, theconnection from the condenser 204 to the evaporator 206 includes theintervening refrigerant pump 208, bypass valve 210, and receiver 212.Where the expansion valves 214 and solenoid valves 216 are consideredseparate from the evaporator 206, the expansion valves 214 and solenoidvalves 216 also intervene.

More than one route may be provided between components or bypassing oneor more components. For example, the bypass valve 210 is connected inparallel with the refrigerant pump 208. As another example, therefrigerant loop splits to flow in parallel through different parts ofthe evaporator 206. The refrigerant may flow only in one or the other orin both where there is a bypass or alternative route. FIG. 2 representsone example, but other fluid interconnections may be provided.

The refrigerant loop 200 of the air cooled chiller 112 interacts withtwo or more other systems. The condenser 204 includes the fan 218 forthermodynamic interaction with air or the ambient environment. Theevaporator 206 includes pipes or channels for interacting with the loadthrough the chilled water pipes 220. Additional, different, or fewersystems interacting with the air cooled chiller 112 may be provided.

The evaporator 206 is formed from one or more parts. As shown in FIG. 2,two parallel evaporator parts are provided. Each part may be activatedor not by the corresponding expansion valves 214. Each part is identicalto the others, but different parts may be sized smaller or larger thaneach other in other embodiments. The division and control through theexpansion valves 214 may allow the evaporator 206 operation to bettermatch the demands of the load (chilled water). The load is more oftennot at the maximum or even near the maximum of the design. As a result,the air cooled chiller 112 may often operate with fewer than all (e.g.,only one) of the evaporator parts at a given time (i.e., run on onecircuit instead of both). Alternatively, the evaporator parts areoperated at a same time for lesser loads where each part is partiallyloaded. Under Demand Flow operation, one evaporator part may operatewith a fully open expansion valve 214, instead of two partially loadedevaporators (i.e., partially closed or open expansion valves 214). Lessrestriction in the flow of refrigerant by the expansion valves 214results in less energy wasted.

The expansion valves 214 are configured by feedback and process controlto fluidly connect and disconnect the evaporator parts individuallybased on chilled water temperature. The chilled water temperature isused as an indication of the load. In other embodiments, otherinformation, such as from the air handling units or condenser fanoperation, is alternatively or additionally used to control theexpansion valves 214. The expansion valves 214 work to maintain constantrefrigerant temperature and pressure.

The solenoid valves 216 are liquid line solenoid valves. In response tocontrol signals, the solenoid valves 216 open or close to enable ordisable evaporator circuits (e.g., sections). The control of thesolenoid valves 216 may be responsive to a temperature of the chilledwater, such as a temperature of the chilled water leaving the evaporator206. The solenoid valves 216 open or close to maintain leaving chilledwater temperature.

In operation, chilled water may enter the evaporator 206, where thechilled water, as “warmed” by the air handling units, transfers heat tothe refrigerant. This evaporates the refrigerant, causing therefrigerant to become refrigerant vapor. The heat transfer from thechilled water cools the chilled water in the pipes 220, allowing thechilled water to return to the air handling units or other load. Toillustrate, 54 degree chilled water may be cooled to 42 degrees bytransferring heat to 40 degree refrigerant within the evaporator 206.The 42 degree chilled water may then be used to cool a building or otherstructures.

In order for the refrigeration cycle to continue, refrigerant vaporproduced by the evaporator 206 is condensed back into liquid form. Thiscondensation of refrigerant vapor may be performed by the condenser 204.As is known, the refrigerant vapor can only condense on a lowertemperature surface. Because refrigerant has a relatively low boilingpoint, refrigerant vapor has a relatively low temperature. For thisreason, a compressor 202 may be used to compress the refrigerant vapor,raising the vapor's temperature and pressure.

The refrigerant travels from the evaporator to the compressor 202. Anynow know or later developed compressor may be used. The compressor 202is a positive displacement (e.g., rotary or reciprocating) or dynamic(e.g., centrifugal or axial) compressor for increasing the pressure ofthe refrigerant vapor by reduction in volume. The refrigerant output ofthe compressor 202 is to the condenser 204. The compressor 202 sendshigh temperature gas to the condenser 204.

The condenser 204 is a heat exchanger, such as a coil with a serpentineor other configuration (e.g., modified crossflow). The surface area isincreased to provide greater exposure to air, the cooling medium. Thecondenser 204 cools the refrigerant vapor down to a liquid, which isthen passed to the evaporator 206 due to pressure from the compressor202.

The increased temperature of the refrigerant vapor from the compressor202 allows the vapor to condense at a higher temperature. For example,without compression, the refrigerant vapor may be at 60 degrees, whereaswith compression the vapor may be at 97 degrees. Thus, condensation mayoccur below 97 degrees rather than below 60 degrees. This is highlybeneficial because it is generally easier to provide a condensingsurface having a temperature lower than the increased temperature of therefrigerant vapor.

The refrigerant vapor enters the condenser 204 where the heat of thevapor may be transferred to air, causing the refrigerant to return to aliquid state. For example, the condenser 204 may comprise a shell andtube design where the air flows through the condenser's tubes. In thismanner, refrigerant vapor may condense on the tubes within thecondenser's shell. After condensing, the refrigerant then returnsthrough a refrigerant line and pressure reducer back to the evaporator206, where the refrigeration cycle continues.

The condenser 204 includes or is adjacent to one or more fans 218 toincrease the flow of air across the condenser 204. By controlling theambient air flow, the fans 218 affect the rate of condensation in thecondenser 204. Since the condenser 204 is exposed to ambient conditionsreflected by the air temperature, any exposure to sunlight, and/or windlevels, the rate of condensation for a given fan speed may be differentdepending on the ambient conditions.

The fan 218 is any now known or later developed condenser fan to causeair flow across or through the condenser 204. The fan 218 is positionedin or adjacent to the condenser 204. Alternatively, the fan 218 isspaced from the condenser 204, such as by a duct for guiding air toand/or from the condenser 204.

The fan 218 includes a blade and a motor. Any blade may be used. Anymotor may be used. In one embodiment, the motor is a variable drive,such as a variable frequency drive (VFD). In response to a controlsignal, such as in response to a frequency, duty cycle, amplitude, orother signal characteristic, the motor controls the speed of the fan218. Change in the speed of the fan 218 causes greater or lesser airflow for greater or lesser condensation in the condenser 204. The changein air flow by the condenser fans 218 may be used to more closelyregulate condenser pressure to maintain a lowest power use of thecondenser fans 218, refrigerant pump 208, and the compressors 202.

In a system for controlling air flow, the condenser fan 218 isresponsive to a control signal feeding back ambient information. Ratherthan measuring temperature of the air, wind speed, and/or sun light, theambient conditions are based on power usage of the chiller. The aircooled chiller 112 draws power from a single electrical circuit, but maydraw power from multiple circuits. The amount of energy used by one ormore, such as all, of the components of the air cooled chiller 112 ismeasured. For example, a power meter is positioned to measure the powerused by the compressor 202, fan 218, and pump 208. As another example,the power meter is configured to measure the power used by the condenserfan 218, refrigerant loop power, and refrigerant pump 208 power. Greaterpower demand indicates less optimal ambient conditions (e.g., more sunlight, less wind, and/or higher temperature air). Using the powerinformation from the power meter, the speed of the fan 218 and resultingair flow may be increased or decreased.

Any power meter may be used, such as a meter for measuring kW/h. Anycharacteristic of energy usage may be measured. Other inputs may be usedfor additional air flow control. For example, the fan speed is varied asa function of both the power and a pressure of refrigerant sensed in thecondenser 204. Any function combining these two inputs to determine fanspeed may be used.

Due to stacking, the liquid refrigerant may build up in the condenser204. Stacking occurs during low ambient temperature operation. The lowambient temperature reduces the rate of condensation in the condenser204. The result of stacking is a buildup of liquid refrigerant andpressure between in the condenser 204 and the evaporator 206.

To avoid stacking in the air cooled chiller, the refrigerant pump 208 isprovided in the condenser 204, in the evaporator 206, or between thecondenser 204 and the evaporator 206. Any refrigerant pump may be used,such as a hermetically sealed liquid pump.

The refrigerant pump 208 is responsive to control signals based onpressure between the condenser 204 and the evaporator 206. A pressuresensor in the condenser 204, such as at a liquid exit of the condenser204, senses the pressure. A pressure sensor in the evaporator 206, suchas at an entrance to the evaporator 206, senses the pressure. The liftor pressure between the condenser 204 and the evaporator 206 isdetermined from the sensed pressures. The control loop activates therefrigerant pump 208 to pump liquid from the condenser 204 in responseto the pressure or lift being above a threshold amount, such as 80 psi.

When stacking does not occur, operation of the refrigerant pump 208becomes inefficient. Thus, the refrigerant pump 208 is connected inparallel with the bypass valve 210. The bypass valve 210 is an expansionvalve, solenoid valve, or other valve responsive to control signals forisolating the refrigerant pump 208. A valve may also be installed inseries with the refrigerant pump 208 to allow flow to or not allow flowto the refrigerant pump 208. The bypass valve 210 operates inconjunction with the refrigerant pump 208. The bypass is closed when therefrigerant pump 208 is operating and open when the refrigerant pump 208is not operating.

The operation of the refrigerant pump 208 forces refrigerant from thecondenser 204 to the evaporator 206. To avoid unwanted increases inpressure caused by the refrigerant pump 208 and to accommodate thevariable amount of refrigerant required by changing ambient and loadconditions, the receiver 212 is provided. The receiver 212 connects withthe refrigerant pump 208. The receiver 212 may be bypassed with therefrigerant pump 208 by the bypass valve 210 or may be connected twoboth paths (i.e., pump 208 and bypass valve 210 as shown in FIG. 2). Inthe example of FIG. 2, the receiver 212 fluidly connects between (i) therefrigerant pump 208 and isolation valve 210 and (ii) the evaporator206.

The receiver 212 is a metal housing for holding liquid refrigerant. Anysize may be used, such as the receiver 212 having 1 ft³ for every 200tons of refrigerant.

The receiver 212 is a tank to hold the liquid refrigerant in varyingload conditions. During operation, the refrigerant pump 208 and receiver212 stop refrigerant from backing up in the condenser 204 and allowbetter low temperature operation. The refrigerant pump 208 and receiver212 may also provide for better control of refrigerant pressure and acloser match of chiller output to load. By reducing or eliminatingstacking, control of the air chiller 112 may more closely meet thedemand by the load. This pressure control is matched to chilled waterrequirements and control requirements. The refrigerant pump 208 andreceiver 212 may also allow the system pressures to be manipulated sothat the expansion valves 214 can operate in an open position.

The air cooled chiller 112 includes various controlled components, suchas controlling the compressor 202, the expansion valves 214, thesolenoid valves 216, the bypass or isolation valve 210, the refrigerantpump 208, and the fan 218. Components of the chilled water plant, suchas a chilled water pump, may also be controlled. Similarly, the controloccurs in response to one or more sensors and/or set points. Thecontroller 302 performs the control of the components. FIG. 3 shows acontroller 302 for controlling and/or receiving information from the aircooled chiller 112 and/or the chilled water system 104/108. Additional,different, or fewer sensors and/or controlled components may beprovided.

The controller 302 is a processor, computer, tablet, applicationspecific integrated circuit, field programmable gate array, analogcircuit, digital circuit, field panel, or other controller. A singlecontroller 302 is shown, but an arrangement of different controllers maybe used. For example, different controllers are provided for differentcomponents (e.g., controller for the fans 218 different than thecontroller for the refrigerant pump 208). The distributed controllersmay communicate for interactive control, may be controlled by a mastercontroller, and/or may operate independent of other control.

The memory 304 is a random access memory (RAM), read only memory (ROM),removable media, flash, solid state, or other memory. The memory 304stores set points, sensor values, control information, and/orinstructions for control by the controller 302. For example, the memory304 is a non-transitory computer readable storage medium for storinginstructions. When the physical controller 302 executes theinstructions, the controls discussed herein are performed.

The controller 302 implements the control functions described herein.For example, the methods are performed under the control of thecontroller 302. Sensor provide information to the controller 302 and/orthe controller 302 queries the sensors. Settings or other informationare known by, provided to, and/or read by the controller 302. Thecontroller 302 generates control signals to cause a change in operationof one or more components.

In one embodiment, the controller 302 is configured by the instructionsto control the air flow caused by the fan 218. The variable frequencydrive 306 of the fan 218 is controlled using energy information from thepower meter 308. The controller 302 controls the air flow caused by thefan 218 as a function of the energy usage of the air cooled chiller 112.Additionally or alternatively, other information than energy usage maybe used to control the air flow, such as using a measure of pressure inthe condenser from the pressure sensor 310.

In another embodiment, the controller 302 is configured to control therefrigerant pump 312 and bypass valve 314. Based on pressure between thecondenser and the evaporator (i.e., lift) sensed by pressure sensors310, the pump 312 is activated or not. The bypass valve 314 iscontrolled to provide bypass, isolating the pump 312, when the pump 312is not operating and is controlled to not allow bypass when the pump 312is operating.

In yet another embodiment, the controller 302 is configured to controlthe expansion valve 318 of the evaporator. In response to refrigerantpressure from a pressure sensor 316 at an exit or within the evaporator,the expansion valve 318 of one or more parallel parts of the evaporatorare altered to further restrict or increase flow of refrigerant. Thechange may be to turn off or on evaporator parts by either allowing ornot allowing refrigerant flow to the part. Since the load dictates theflow of chilled water, the load influences the pressure in theevaporator. The controller responds with control of the expansion valves318.

The expansion valve 318 is controlled to set the refrigeranttemperature. In addition or as an alternative to control based onpressure, information from the air handling unit or other load is used.For example, a valve position 324 with or without other parameters(e.g., air temperature or difference of air temperature from set pointof the air handling unit 326; importance of the air handling unit 328;and/or valve size of the air handling unit 330) are used to control theexpansion valves 318. Where increased load is indicated by the valveposition and/or other parameters, additional parts of the evaporator areactivated and/or the amount of flow through the parts is altered toprovide refrigerant at the desired temperature. The expansion valvesoperate in response to the load.

The controller 302 is configured to control the solenoids 322 of theevaporator. In response to chilled water temperature from a sensor 320at the exit, entrance, or other location of chilled water in theevaporator, the controller 322 increases or decreases flow ofrefrigerant to the evaporator or part. By increasing or decreasing flow,the temperature of the chilled water may be controlled.

In another embodiment, the controller 302 is configured to control achilled water pump 120. The flow of chilled water through the evaporatoris controlled. Any sensors and/or information may be used to control thewater flow. In the example of FIG. 3, the valve position (e.g., size ofaperture, amount open or closed) of an air handling unit, a temperatureof the air exiting the air handling unit (e.g., difference from setpoint), an importance of the air handling unit, and the valve size ofthe valve of the air handling unit are used. Additional, different, orfewer types of information may be used. For example only one (e.g.,valve position), two or more, or three or more of the types ofinformation are used. Any function may be used for combining theinformation. For example, the valve position (e.g., 80% open) is astarting value, which is then modified by other information. The 80% isdecreased for smaller valves or increased for larger valves; theimportance increases or decreased the value; and lower temperaturesdecrease the value while greater temperature increases the value. Thealteration of the value is not for altering the actual valve position,but is instead used to alter the control value. Where multiple airhandling units are serviced, the values for each air handling unit arecombined, such as using a high weighted mean (e.g., average of theaverage and the highest value). Any function for combination of valuesfrom different air handling units may be used.

The resulting indication of load is used in any control sequence tocontrol the chilled water flow. For example, a proportional integralderivative (PID) control loop is used. In one embodiment, a variablepressure curve is used to control the flow of chilled water based on thevalue calculated from the load devices.

In addition to or an alternative to chilled water flow, the refrigeranttemperature is controlled based on information from the air handlingunit. For example, the position of a valve 324 of the air handling unitis used as an input for control of the expansion valves 318. Otherparameters from the air handling unit and/or the air cooled chiller 112(e.g., evaporator pressure 316) may also be used for control of theexpansion valves 318. The expansion valve 318 position and/or activationof the different parts are controlled based on the load (air handlingunit information) and/or pressure in the refrigerant loop.

The air cooled chiller 112 is used in the chilled water plant. Thecontrol operation uses demand flow in variable air volume or otherenvironments. Air cooled chillers 112 may be used with any chilled waterplant. In some embodiments, the air cooled chillers 112 are used in datacenters where space for water cooling is not available. In otherembodiments, air cooled chillers 112 are used due to lesser initial costthan water cooled chillers. In yet other embodiments, regulations mayprevent use of water cooled chillers, so the air cooled chiller 112 isused instead.

The overall control is synchronized between the different components.The synchronization is through interaction of the control functions bythe controller. Alternatively or additionally, synchronization occursthrough the refrigerant loop. For example, chilled water flow iscontrolled. As a result, the evaporator pressure changes. The controlroutines relying on evaporator pressure (e.g., operation of therefrigerant pump, operation of the evaporator expansion valves, and/orselection of evaporator parts) measure the pressure and reactaccordingly. Through this synchronization, an operational strategy toprovide efficient cooling is implemented by the controller 302.

Operational strategy refers to the principles, operations, andalgorithms applied to the chilled water plant, air cooled chiller, andcomponents thereof to achieve Demand Flow's benefits to plant energyusage and cooling capacity. Application of the operational strategy mayreduce or eliminate Low Delta T by operating chilled water plantcomponents at or near design Delta T, regardless of load conditions.This in turn optimizes energy usage and deliverable capacity for chilledwater plant components and the plant as a whole.

FIG. 4 shows a generalized control method. FIGS. 5-7 and 9-10 representexample general control strategies or methods for specific controlaspects. FIGS. 11-12 represent some generalized control principles.FIGS. 14-19 are example embodiments of specific control strategies forspecific control aspects. The various set points, increment sizes,measures, thresholds, calculations, and/or times may have differentvalues.

FIG. 4 is a flow diagram illustrating exemplary operations which may beperformed by a controller 302 to perform the operational strategy. Someacts described herein may be performed in different order than describedherein, and there may be fewer or additional steps in variousembodiments corresponding to various aspects of the operationalstrategy.

In the embodiment shown, sensor information is received at act 1104. Forexample, sensor information regarding pressure, temperature, orderivations thereof may be received. Refrigerant and/or chilled watercharacteristics may also be received. Also, operating characteristics,such as the position of chilled water valves at air handlers, the speedor output of VFDs, the speed or flow rate of pumps, as well as otherinformation may be received.

At act 1108, based on the information received in step 1104, thecontroller may determine whether to increase or decrease flow rate ofone or more pumps, alter valve position, and/or alter fan speed, such asto maintain a Delta T near or at design Delta T. The operation of anytype of component may be controlled.

At act 1112, an output may be provided, such as to a VFD or other pumpcontroller, valve, or even to a pump directly to increase or decrease asdetermined in act 1108.

The acts are performed in response to triggers, such as in response to achange in an input. Alternatively or additionally, the acts areperformed periodically. For example, the control routine for a givencomponent, for synchronizing the components, and/or for operating theair cooled chiller is performed once every 5 minutes. Sensors arequeried or the most recent readings examined to determine any newsettings for operating the components at the beginning of each period.

The operational strategy may also include one or more set point resets,such as resetting the refrigerant temperature setting at the evaporator.A set point reset may be used to increase or decrease cooling output,such as by increasing or decreasing refrigerant temperature. This mayoccur in situations where cooling demand cannot be met by operating achilled water plant at a particular setting. For example, if coolingdemand cannot be met, a set point reset may be used to reset the currentrefrigerant temperature maintained by the operational strategy to a newvalue. To illustrate, the refrigerant temperature maintained by anoperational strategy at an output of the evaporator may be reset from 42degrees to 40 degrees. To produce this lower value, the pressure ascontrolled by the opening and number of open expansion valves isaltered. For example, an additional part is operated and/or the flow ofrefrigerant is slowed. The control of refrigerant temperature may besynchronized with the flow of chilled water and/or the demand by airhandling units. For example, the flow rate of chilled water may beincreased to maintain the new Delta T value across one or more chilledwater plant components. The increased flow rate provides additionalchilled water to chilled water plant components, which in turn providesincreased cooling output to meet demand. For example, increased chilledwater flow to air handlers would give the air handlers additional coolair capacity.

FIG. 5 shows one embodiment of a method for controlling and/or using anair cooled chiller. In particular, control of the refrigerant pump toavoid stacking is shown. The method is performed by a controller usingsensors, settings, and/or controllable components (e.g., pumps, motors,and/or valves). The method is performed in the order shown or adifferent order. For example, the condensing, evaporating, andcompression through pressure control of acts 502, 504, and 506 areperformed at a same time in an on-going manner. Acts 508 may beperformed periodically or at triggered times during the on-goingoperation of the chiller. The storage in the receiver of act 510 alsooccurs in an on-going manner or may only occur when act 508 also occurs.

In act 502, refrigerant is condensed in a condenser. By exposingrefrigerant vapor under pressure to cooler temperatures, the refrigeranttransitions to a liquid state by condensation. Air is passed by or incoils separating the air from the refrigerant. The air is cooler due tothe compression or pressure of the vapor, resulting in condensation.

In act 504, refrigerant is evaporated in an evaporator. Liquidrefrigerant is exposed to heat from a load. The heat transfers to therefrigerant, causing a transition to vapor.

The amount of condensation and/or evaporation is controlled by controlof the pressure of the refrigerant. In act 506, a compressor increasesthe density of the refrigerant vapor, causing a rise in the refrigeranttemperature as well as pressure within the refrigerant loop. Theoperation of the compressor controls, in part, the pressure of therefrigerant within the compressor and elsewhere in the refrigerant loop.The refrigerant pump, expansion valve settings, and/or solenoid valvesettings may also control pressure, such as by restricting flow.

In act 507, pressure is sensed. The pressure in the condenser, in theevaporator, and/or in other parts of the refrigerant loop is sensed. Inone embodiment, the pressure lift or pressure difference between thecondenser and the evaporator is sensed. The pressure between thecondenser and evaporator of liquid and/or vapor refrigerant is sensed.

In act 508, liquid refrigerant is pumped. The refrigerant is drawn fromthe condenser and pumped towards the evaporator. Any amount or pumpingforce may be used. In one embodiment, the control is whether to pump ornot. In other embodiments, the control may also include how forcefullyto pump (e.g., pump speed or pressure).

The pumping occurs when the lift or pressure between the condenser andevaporator is above a threshold. The pressure sensed in act 507 is usedto control the pumping. For example, if the pressure difference is 80psi or greater, then the pumping of act 508 is performed. Otherwise,pumping is not performed and the refrigerant bypasses the pump. The pumpis isolated for the bypass by opening a bypass valve.

In act 510, the refrigerant is stored in a receiver. During pumping, thereceiver acts to protect the evaporator from undesired refrigerantpressure. Some of the refrigerant is stored, at least temporally, in thereceiver. During bypass operation, the refrigerant passes through thereceiver. The storage is temporary. Any overflow of refrigerant passesto the evaporator. The pressure of the refrigerant causes flow throughthe tank to the evaporator.

FIG. 6 shows one embodiment of a method for controlling and/or using anair cooled chiller. In particular, control of the condenser fan in anair cooled chiller is shown. The method is performed by a controllerusing sensors, settings, and/or controllable components (e.g., pumps,motors, and/or valves). The method is performed in the order shown or adifferent order. Acts 600, 602 and 604 are performed periodically, suchas every five minutes.

In act 600, the pressure in the condenser is measured. The vaporpressure of the condenser is measured, such as the pressure of therefrigerant vapor input from the compressor. Since the condensationactions reduce the pressure, the vapor pressure is responsive to theamount of condensation. The pressure is measured as feedback on thecondensation.

In act 602, the power used by the air cooled chiller is measured. Apower meter measuring the electrical power used by one or morecomponents of the chiller and/or chilled water plant is measured. Forexample, a power meter connection to mains or a power meter insertedbetween the electrical source and the chiller components measures theenergy used. The condenser fan, compressor, and refrigerant pumpsrepresent steady or periodically used consistent power drains. Bymeasuring an average power over time, such as over minutes or hours, theburden on the air cooled chiller is indicated. Since the ambientconditions, such as air temperature, sunlight, and/or wind, moststrongly affect the amount of power used, the power measure may be afeedback indication of ambient conditions.

In act 604, the fan speed of the condenser fan is adjusted. The fanspeed is increased, decreased, or maintained. Where more power is beingused, then the fan speed is increased to provide more air flow throughthe condenser, resulting in more condensation. For example, more powerby the chiller is used where the ambient temperature increases orsunlight is shining on the condenser. The fan speed is increased toprovide more air flow in these conditions. When the sun sets or goesbehind clouds, less power is used so the fan speed may be decreased.Where less power is being used, then the fan speed may be decreased toprovide less air flow, resulting in less condensation. The condenserpressure may be used separately to further adjust or may be used incombination with power using any function to determine when and by howmuch to adjust.

Any step size or adjustment amplitude may be used. For example, the fanspeed is increased by 5% for each adjustment and the inputs are used todetermine whether to adjust. As another example, the inputs are used todetermine how much to adjust or to determine a fan speed setting thatmay or may not be different than previously used.

By adjusting the air flow, the condensation rate changes or stays thesame where ambient condition changes would result in less or morecondensation. The air flow affects the energy exchange from therefrigerant vapor to the air. Controlling the fan speed may result inless power usage by the fan and/or other components of the chiller. As aresult of changing the air flow, the pressure of the refrigerant vaporin the condenser changes. The change may cause less or more liquidrefrigerant to flow to the evaporator.

FIG. 7 shows one embodiment of a method for controlling and/or using anair cooled chiller. In particular, control of the chilled water flow ina chilled water system using an air cooled chiller is shown. The controlof the chilled water flow using air handling unit valve setting is astaught using Demand Flow in U.S. Published Patent Application No.2011/0301766. The method of FIG. 7 adds consideration of other loadinformation, such as including one or more of air temperature at the airhandling unit, size of the valve at the air handling unit, and priorityof the air handling unit relative to other air handling units. By usingthe valve setting or position in addition to other load information,more precise or optimized control of chilled water flow is provided.

The method is performed by a controller using sensors, settings, and/orcontrollable components (e.g., pumps, motors, and/or valves). The methodis performed in the order shown or a different order. Acts 702 and 704are performed at a same time or in any order.

In act 702, the setting of valves for one or more air handling units isdetermined. The determination may be by looking up a control value sentto the air handling unit. Alternatively, the determination is byreceiving an indication from the air handling unit of the setting and/orby querying the air handling unit.

The valve setting represents the aperture size or amount of open orclosure of the valve. In one embodiment, the value of the valve settingis a percentage open (e.g., 80% open). Other values may be used, such asa measure of flow area and solenoid control signal. Representativevalues may be used, such as mapping the valve setting to a scale withany dynamic range.

In act 704, other parameters are determined. The determination is bylook-up, receipt, or query. Any other parameters reflective of the loadconditions may be used. For example, one, two, or all three of airtemperature (e.g., difference from set point), size of a valve, and/orpriority of the air handling unit is determined.

The determination may be based on the parameter being determined. Forexample, a memory local to the controller stores an indication of theimportance of each air handling unit. A ranking system may be provided,such as 1-10 where 1 is the highest priority and 10 is the leastpriority. The rank is established by design or user input. As anotherexample, the size of the valve of the air handling unit is indicated bythe air handling unit or is stored based on design or user input. Anymeasure of size may be used. The air temperature may be sensed at theair handling unit. The air temperature itself is provided to thecontroller or the air handling unit determines a difference between atemperature set point and a temperature output by the air handling unit.The difference is sent as an indication of air temperature at the airhandling unit to the controller.

In act 706, the flow rate of the chilled water through the air handlingunit and the air cooled chiller is controlled based on the valve settingand/or other parameters. The flow rate may be increased, decreased, orkept the same, depending on the inputs. The inputs used are the valvesetting and one or more other parameters.

Any function combining the inputs may be used. In one embodiment, theinputs are combined by alteration of a base value. For example, thevalve setting is a base value. The values of the other parameters areused to increase or decrease this base value. The base value is adjustedto account for other load information. Any amount of adjustment may beprovided. The amount may be binary, such as adjusting up or down if thevalue of the parameter is beyond a range. The amount may be a functionof the value of the other parameter, such as a linear or non-linearadjustment. For example, the amount of adjustment may be different forair handler units of different priority. Higher priority results in anadjustment upward, and lower priority results in an adjustment downward.Average priority results in no adjustment. As another example, greatertemperature difference between supplied air and the air temperature setpoint results in greater adjustment. The size of the valve may behandled like the priority. Other mapping may be used.

The resulting value is used to control the chilled water flow rate.Where more cooling is desired, reflected by greater openings of thevalve as modified by other parameters, then more chilled water flow maybe desired. The chilled water pumping is increased to increase the flow.Where less cooling is needed, then less flow of the chilled water may bedesired. By operating the pump less or providing less flow, less energymay be consumed by the pump.

Since a given air cooled chiller and/or chilled water line may use morethan one air handling unit, the control of the flow rate of the chilledwater may be set based on the inputs from multiple air handling units.Any combination of inputs may be used. In one embodiment, a highweighted mean is used. The combined values for each air handling unitare averaged across air handling units. The average of the average andthe highest of the combined values is used as the control value. Basedon the control value, the flow of chilled water is increased by a setamount or an amount that varies depending on the value. A PID or othercontrol loop may be used.

In one embodiment, the chilled water flow is varied using a variablepressure curve logic through feedback from the air handling units. FIGS.8, 9 and 11 illustrate, respectively, the exemplary chilled water plant2700 configured to operate in conjunction with a demand flow device2502, and algorithms and processes for determining optimal pressuresetpoints associated with the condenser and evaporator operable therein.FIG. 12 illustrates an algorithm and process for calculating theoperating pressure exponent utilized by the condenser and evaporatorroutines discussed and disclosed herein.

FIG. 8 illustrates the exemplary chilled water plant 2700 including aprimary loop 104 and the secondary loop 108. The components of theexemplary chilled water plant are shown herein coupled to or incommunication with the controller 1000 and the demand flow device 2502.The chiller 112, in fluid communication with the primary loop 104,comprises the condenser 2712, the compressor 2714 and the evaporator2710 coupled via refrigerant lines 2736 and expansion valve 2738. Theevaporator 2710 may be connected to a primary or other loop of a chilledwater plant by one or more chilled water lines 2732. The demand flowdevice 2502, via the controller 1000, executes the demand flow controlroutine to control the operation of, for example, the compressor 2714, acondenser fan 2730, and water pumps 2716 and 2720 operable in theprimary and secondary loops, respectively.

In this exemplary embodiment, the demand flow control routine receives,via the processor 1004, memory 1012 and input 1020; sensor data andcommunicates each of the subsequently calculated optimal pressurecontrol setpoints to the compressor and water pumps operable within thechilled water plant 2700. FIG. 8 depicts the demand flow device 2502communicating an optimal pressure setpoint A to the water pump 2716.Similarly, the condenser fan 2730 controlling or adjusting the pressurewithin the chiller 112 receives an optimized pressure setpoint B.Secondary and tertiary water pumps such as the exemplary water pump 2720will likewise receive optimized pressure setpoints (represented by thereference identifier “C”). By controlling the delta T between thechiller 112 and the primary and secondary fluid loops, the operation ofthe air handling unit 2746 may be controlled and adjusted to similarlyoptimize their performance and energy usage as indicated by thereference identifier “E.”

Sensor data related to measured and operational parameters occurringthroughout the chilled water plant 2700 may be detected by the sensors2714 a, 2714 b, 2730 a, 2730 b, 2716 a, 2716 b, 2720 a and 2720 bdeployed at the appropriate supply and return points adjacent to thecompressor 2714, the condenser air fan 2730, and the water pumps 2716and 2720, respectively. The detected sensor data may, in turn, becommunicated to the input 1020 for use by the processor 1004 (and storedwhen applicable in the memory 1012 and specifically the memory stack2506).

In this manner, the demand flow device 2502 evaluates each of thecomponents operable within the chilled water plant 2700 based on designcharacteristics, measured operational performance and current loadrequirements. The demand flow control routine 2600 operable within thedemand flow device 2502 then calculates in real-time or near real-time(e.g., every 5 minutes) an optimal pressure setpoint for each of thecomponents to control the flow rate and ultimately regulate the delta Tacross each of the components in order to implement the demand flowvariable pressure curve logic operational strategy.

Demand flow variable pressure curve logic (VPCL) as implemented by thedemand flow control routine 2502 optimizes the total system energy ofthe chilled water plant 2700 by synchronizing the operation of theindividual components operating therein. In particular, the individualcomponents are synchronized to a calculated efficiency curve relative tothe current environmental/load condition sensed via the sensors 2714 a,2714 b, 2730 a, 2730 b, 2716 a, 2716 b, 2720 a and 2720 b.

In order to maximize user comfort and optimize system efficiency, thedemand flow control routine 2502 utilizes a comprehensive optimizationalgorithm and process to minimize the energy usage of the chiller 112.Demand flow variable pressure curve logic varies provides the mechanismby which the energy usage is controlled by optimizing the pressuresetpoints of the compressor 2714, and pumps 2716, 2720 and 2730 which,in turn, allows for the control of the water temperature and flow ratethroughout the chilled water plant 2700.

FIGS. 9 and 11 illustrate exemplary algorithms and processes fordetermining optimal pressure setpoints associated with the condenser andevaporator that may be implemented by the demand flow device 2502 andthe demand flow control routine. FIG. 9 is an operational flowchart 2800of the procedures, steps and tasks that may be implemented by the demandflow control routine, and more particularly the demand flow condenserroutine portion of the demand flow control routine, in order to optimizethe performance and efficiency of the condenser 2712 (see FIG. 8).

An initial step or task undertaken to utilize the principles of thedisclosed demand for variable pressure curve logic is to identify andreview the design parameters of the one or more components operatingwith interconnection with the chilled water plant 2700. For example,prior to beginning implementation of the disclosed optimizationalgorithm and routine, the user or designer may input or provide the oneor more design parameters to the memory 1012 or any other accessibledatabase or storage location (block 2802). The design parameters mayinclude: the design condenser flow rate (GPM); the condenser designpressure differential (PS ID); the design condenser capacity (Tons). Thedesign parameters may further include the baseline or design chilledwater delta T that represents the full load chilled water delta Tcalculated at the time commissioning of the chilled water plant 2700.These initial design parameters provide the demand flow processor, andmore specifically the demand flow condenser routine, with a baselineperformance envelope against which the condenser 2712 may be evaluated.

The demand flow control routine and the demand flow condenser routineare further configured, as shown at block 2804, to empirically calculatean operating pressure exponent (P Exponent) based on measured parametersof the condenser 2712 operating in the chiller 112. In this exemplaryembodiment, the operating pressure exponent is calculated according tothe formula:P Exponent=Ax ² +Bx+Cwhere x is the delta P or pressure change measured across the condenser2712 (see block 2804) and the constants A, B and C are calculated foreach chilled water plant 2700. Other formulas may be used. Thisrelationship is discussed in more depth in connection with FIG. 12. Inparticular, FIG. 12 illustrates a pressure exponent algorithm androutine 3000, that may be utilized to empirically derive discretepressure exponents and an overall pressure exponent curve that fits orotherwise connects each of the discrete pressure exponents. The overallpressure exponent curve, and more particularly, the equation describingthe overall exponent pressure curve, in turn, is utilized by thecondenser routine to determine the operating pressure exponent shownabove.

As the pressure exponent routine 3000 initializes, the delta P orpressure differential (PSID) across the condenser 2712 is measured at avariety of fan speeds specified at the condenser fan 2730. For example,the pressure exponent routine 3000 records the fan speed in Hertz (Hz)and the pressure differential (PSID) across the condenser 2712 when thecondenser fan 2730 is operating at a partial load value (PLV)corresponding to 25%, 50%, 75% and 100% pumping capacity (block 3002).The pressure exponent for each discrete partial load value (PLV) iscalculated as a function of the maximum operation pressure differential(PSID) of the system, the fan speed in Hertz and the measured operatingpressure differential at a given PLV. The formula for calculating one ofthe discrete P Exponents for a given PLV is:

${{PLV}\mspace{14mu}{Operating}\mspace{14mu}{PSID}} = {{{Max}.\mspace{14mu}{Operating}}\mspace{14mu}{PSID}*{\left( \frac{{Pump}\mspace{14mu}{Speed}\mspace{14mu}({Hz})}{60} \right)^{P\mspace{14mu}{Exponent}_{PLV}}.}}$Other formulas may be used.

The maximum operating pressure differential (PSID) is a known designvalue, and the fan speed (Hz) and the operating pressure differential(PSID) are measured and/or empirically derived values. In this way, itis possible to calculate a discrete pressure exponent (P Exponent_(PLV))for each set of variables associated with a given partial load value(PLV). Stated another way, by balancing the left and right sides of theabove-formula, the discrete pressure exponent (P Exponent_(PLV)) can bederived for a given partial load value (e.g., 25%, 50%, 75% and 100%capacity) and measured operating pressure differential at the given PLV(block 3004). The resulting the discrete pressure exponents (PExponent_(PLV-25%), P Exponent_(PLV-50%), P Exponent_(PLV-75%), and PExponent_(PLV-100%)) may be plotted according to the measured operatingpressure differential in order to define the overall pressure exponentcurve. The equation describing the overall exponent pressure curve(block 3006) can be derived based on these plotted values.

Once the equation describing the overall exponent pressure curve hasbeen derived, the pressure exponent routine 3000 completes and returnsto the operational flowchart 2800. At this point, the equationdescribing the overall exponent pressure curve may be utilized tocalculate the operating pressure exponent for any given pressuredifferential (PSID). An exemplary overall exponent pressure curveequation may be defined as:P Exponent=−0.00003x ²+0.0031x+1.9358where x is the delta P or pressure change measured across the condenser2712 (see block 2804) and the constant A equals −0.00031, the constant Bequals 0.0031 and the constant C equals 1.9358.

This portion of the demand flow condenser routine may be considered thedesign or configuration portion of the routine while the remaining stepsand operations may be characterized as the run or operational portion ofthe routine.

The demand flow condenser routine 2612 utilizes the sensors 2712 a and2712 b to detect and measure the pressure differential (PSID) across thecondenser 2712. In this embodiment, the sensors 2712 a and 2712 b may bewater immersion sensors specified with a suitable range (e.g. 20°-120°F.) to measure the refrigerant delta T. In other embodiments andconfigurations, the temperature sensors may be high or higher accuracysensors (±0.5° F.) arranged to detect minor variations in the condenserand/or chilled water flow throughout portions of the chilled water plant2700. Moreover, the demand flow control routine sensors 2712 a and 2712b and/or components or subsystems thereof may be configured and arrangedto measure a chilled water supply (CWS) temperature (sensor 2712 a) andthe chilled water return (CHR) temperature (sensor 2712 b) associatedwith the condenser 2712 (at block 2806).

The demand flow condenser routine may subsequently utilize the detectedand measured pressure and temperature information to calculate a fanspeed for the condenser 2712 (at block 2808). In particular, the demandflow processor 2502 accesses the values stored in, for example, thememory 1012 if previously accessed and stored locally) as directed bythe demand flow control routine. The condenser fan speed is calculatedaccording to the formula:

${{{Meas}.\mspace{14mu}{Condenser}}\mspace{14mu}{Flow}\mspace{14mu}{Rate}} = {{T\&}B\mspace{14mu}{speed}*\left( \frac{{{Meas}.\mspace{14mu}{Condenser}}\mspace{14mu}\Delta\; P\mspace{14mu}({PSID})}{{Design}\mspace{14mu}{Condenser}\mspace{14mu}\Delta\; P\mspace{14mu}({PSID})} \right)^{.5}}$where the test & balance (T&B) speed represents the actual speed of thefan. This is usually different that the “design” or full-rated speed dueto the variance in manufacture and installation. As previouslydiscussed, the sensors 2712 a and 2712 b may be high accuracy pressuresensors arranged to measure the supply pressure (via sensor 2712 a) andthe return pressure (via 2712 b). The difference between the measuredsupply and return pressures represents the pressure differential or lossacross the condenser 2712.

Upon determination of the measured condenser fan speed, the demand flowcondenser routine and the demand flow control routine (as shown at block2810) calculate the current condenser output capacity (Tons). Thecurrent condenser capacity may be calculated according to the formula:

${{Condenser}\mspace{14mu}{Capacity}\mspace{14mu}({Tons})} = {{{Meas}.\mspace{14mu}{Condenser}}\mspace{14mu}{Flow}\mspace{14mu}{Rate}*\left( \frac{{CWR} - {CWS}}{24} \right)}$

In this way, the demand flow condenser routine portion of the demandflow control routine can empirically calculate the output capacity eachcondenser 2712 operating in conjunction with the chilled water plant2700.

The demand flow control routine and the demand flow condenser routinemay, in turn, utilize the results from the previous steps andcalculations to determine a virtual delta T (see block 2812). Thevirtual delta T represents a hypothetical or equivalent chilled waterdelta T that would be present if a constant volume pumping algorithmwere utilized under the current operating conditions. The virtual deltaT may be calculated according to the formula;

${{Virtual}\mspace{14mu}{Delta}\mspace{14mu} T} = {{Design}\mspace{14mu}{Delta}\mspace{14mu} T*{\left( \frac{{Condenser}\mspace{14mu}{Capacity}\mspace{14mu}({Tons})}{{Design}\mspace{14mu}{Condenser}\mspace{14mu}{Capacity}\mspace{14mu}({Tons})} \right).}}$

The demand flow control routine and the demand flow condenser routinemay, at block 2814, utilize the results and information from one or moreof the preceding algorithm steps to determine a pressure curve set pointfor the condenser 2712. The pressure curve setpoint may be determinedaccording to the formula:

${{PC}\mspace{14mu}{Setpoint}} = {{Design}\mspace{14mu}{Condenser}\mspace{14mu}\Delta\; P\mspace{14mu}({PSID})*\left( \frac{{Virtual}\mspace{14mu}{Delta}\mspace{14mu} T}{{Baseline}\mspace{14mu}{Delta}\mspace{14mu} T} \right)^{P\mspace{14mu}{Exponent}}}$where the baseline delta T represents the full load delta T chosen orselected at the time of commissioning of the chilled water plant 2700.The baseline delta T may (and often will) correspond to the design deltaT. Other formulas may be used.

In another embodiment, the demand flow control retained and the demandflow condenser routine, at block 2814, may utilize the parameters andinformation from one or more of the preceding algorithm acts inconjunction with the measured active pressure differential (PSID) valueto directly determine the pressure curve set point for the condenser2712. The pressure curve setpoint according to this alternate embodimentmay be determined according to the formula:

${{PC}\mspace{14mu}{Setpoint}} = {{Active}\mspace{14mu}{Condenser}\mspace{14mu}\Delta\; P\mspace{14mu}({PSID})*{\left( \frac{{Measured}\mspace{14mu}{Delta}\mspace{14mu} T}{{Design}\mspace{14mu}{Delta}\mspace{14mu} T} \right)^{P\mspace{14mu}{Exponent}}.}}$Other formulas may be used.

The calculated pressure curve set point may, in turn, be communicatedfrom the demand flow control routine and demand flow processor via thecommunications module to the fan 2730 (see block 2816). Duringoperation, if the temperature of the condenser air detected by thesensor 2712 b rises above or exceeds a threshold level corresponding to,for example, the supply chilled water temperature setpoint plus a smalldeadband temperature (e.g. 1° F.), then the demand flow device 2502 andthe demand flow control retained initiates an override to linearlyincrease or ramp up the VFD associated with the condenser fan 2730 tofull speed. When the temperature of the chilled water exceeds thethreshold, a loss of communication occurs as the temperature surpassesthe sensor's ability to detect the temperature. The subsequent increasedflow rate provided by the ramped up VFD limits additional heat transfer,thereby causing the temperature to decline and communications to berestored. The VFD may further be programmed to initiate a decelerationramp after, for example, 15 min. at full speed.

Referring to FIG. 10, the refrigerant temperature set point for theevaporator is controlled. The refrigerant set point in the evaporatorwill change. The expansion valve resets the evaporator set point. Theset point is varied to meet load conditions in conjunction with varyingthe set point of the chilled water temperature and to allow for properrefrigerant flow from the condenser. To be responsive to the load, therefrigerant set point is a function of information from the air handlingunit, such as the setting for the air handling unit valve position.Other parameters are optionally used, such as the air temperature (e.g.,difference in supplied verses desired), importance of the air handlingunit, and/or valve size.

FIG. 10 shows one embodiment of a method for controlling and/or using anair cooled chiller. In particular, control of the evaporator temperatureset point in an air cooled chiller is shown. The control of therefrigerant set point is as taught using Demand Flow in U.S. PublishedPatent Application No. 2011/0301766, but with added consideration of theload (e.g., air handling unit) information. The method of FIG. 10 addsconsideration of the load information, such as including one or more ofvalve position, air temperature at the air handling unit, size of thevalve at the air handling unit, and priority of the air handling unitrelative to other air handling units. By using the load information,more precise or optimized control of chilled water flow is provided.

The method is performed by a controller using sensors, settings, and/orcontrollable components (e.g., pumps, motors, and/or valves). The methodis performed in the order shown or a different order. Acts 1002 and 1004are at a same time or in any order. Additional, different, or fewer actsmay be performed. For example, act 1004 is not performed.

In act 1002, the setting of valves for one or more air handling units isdetermined. The determination may be by looking up a control value sentto the air handling unit. Alternatively, the determination is byreceiving an indication from the air handling unit of the setting and/orby querying the air handling unit.

The valve setting represents the aperture size or amount of opening orclosure of the valve. In one embodiment, the value of the valve settingis a percentage open. Other values may be used, such as a measure offlow area and solenoid control signal. Representative values may beused, such as mapping the valve setting to a scale with any dynamicrange.

In act 1004, other parameters are determined. The determination is bylook-up, receipt, or query. Any other parameters reflective of the loadconditions may be used. For example, one, two, or all three of airtemperature, size of a valve, and/or priority of the air handling unitis determined.

The determination may be based on the parameter being determined. Forexample, a memory local to the controller stores an indication of theimportance of each air handling unit. A ranking system may be provided,such as 1-10 where 1 is the highest priority and 10 is the leastpriority. The rank is established by design or user input. As anotherexample, the size of the valve of the air handling unit is indicated bythe air handling unit or is stored based on design or user input. Anymeasure of size may be used. The air temperature may be sensed at theair handling unit. The air temperature itself is provided to thecontroller or the air handling unit determines a difference between atemperature set point and a temperature output by the air handling unit.The difference is sent as an indication of air temperature at the airhandling unit to the controller.

In act 1006, the refrigerant temperature set point is controlled basedon the valve setting and/or other parameters. The temperature set pointmay be increased, decreased, or kept the same, depending on the inputs.Any function combining the inputs may be used. In one embodiment, theinputs are combined by alteration of a base value, as discussed above.

The resulting value is used to control the refrigerant temperature. Byopening or closing expansion valves, the dwell time of refrigerant inthe evaporator is changed. The pressure varies. The expansion valves arecontrolled to alter the amount of energy transfer, and thus therefrigerant temperature. For example, more parts of the evaporator areactivated to provide a greater increase in temperature of therefrigerant. To provide for a relatively lower refrigerant temperature,one or more parts of the evaporator are turned off (e.g., expansionvalves closed). Other than binary control (e.g., expansion valvespartially open) may be used.

Since a given air cooled chiller and/or chilled water line may use morethan one air handling unit, the control of the flow rate of the chilledwater may be set based on the inputs from multiple air handling units.Any combination of inputs may be used. In one embodiment, a highweighted mean is used. The combined values for each air handling unitare averaged across air handling units. The average of this average andthe highest of the combined values is used as the control value. Basedon the control value, the expansion valves and corresponding refrigeranttemperature are varied by a set amount or an amount that variesdepending on the value. A PID or other control loop may be used.

In one embodiment, the refrigerant temperature is varied using variablepressure curve logic through feedback from the air handling units. Bycontrolling pressure, the refrigerant evaporates at a specifictemperature. This refrigerant gas also changes temperature somewhat.Both the evaporation of the liquid refrigerant and the warming of therefrigerant gas are used to cool the water down.

FIG. 11 illustrates an example algorithm and process for determiningoptimal refrigerant temperature setpoint associated with the evaporator.FIG. 12 illustrates an algorithm and process for calculating theoperating pressure exponent utilized by the condenser and evaporatorroutines discussed and disclosed herein.

FIG. 11 is an operational flowchart 2900 for implementing implementedvariable pressure curve logic operational strategy by the demand flowcontrol routine 2600 in order to optimize the performance and efficiencyof the evaporator 2710 (see FIG. 8).

In the design portion or stage of the demand flow evaporator routinereceives and organizes one or more design parameters in the memory 1012or any other accessible database or storage location (as shown at block2902). The design parameters may, as previously discussed, include: adesign evaporator flow rate (GPM); an evaporator design pressuredifferential (PSID); the design evaporator capacity (Tons); a baselineor design chilled water delta T that represents the full load chilledwater delta T calculated at the time commissioning of the chilled waterplant 2700. In another embodiment, the organized design parameters canbe supplemented with or augmented by a current or active pressuredifferential (PSID) value measured across the evaporator 2710. Utilizingthese design and/or measured parameters, the performance envelopeagainst which the evaporator 2710 is to be evaluated may be establishedby the demand flow processor and the demand flow condenser evaporatorroutine.

An evaporator-specific pressure exponent (P Exponent) may beempirically-derived and calculated by the demand flow control routineand the demand flow evaporator routine. The pressure exponent (PExponent) may be based on measured parameters of the evaporator 2710operating in the chiller 112. The pressure exponent may be calculatedaccording to the formula:P Exponent=Ax ² +Bx+Cwhere x is the delta p or pressure change measured across the evaporator2710 (see block 2904) and the constants A, B and C are calculated foreach chilled water plant 2700. Other formulas may be used. As previouslydiscussed in connection with the condenser 2730, FIG. 12 illustrates apressure exponent algorithm and routine 3000 that may be utilized toempirically derive discrete pressure exponents and an overall pressureexponent curve that fits or otherwise connects each of the discretepressure exponents. The overall pressure exponent curve, and moreparticularly, the equation describing the overall exponent pressurecurve, in turn, is utilized by the evaporator routine to determine theoperating pressure exponent shown above.

Once the equation describing the overall exponent pressure curve hasbeen derived, the pressure exponent routine 3000 completes and returnsto the operational flowchart 2900. At this point, the equationdescribing the overall exponent pressure curve may be utilized tocalculate the operating pressure exponent for any given pressuredifferential (PSID) as discussed previously.

The demand flow evaporator routine 2610 begins the operational or runportion of the routine by utilizing the sensors 2710 a and 2710 b todetect and measure the pressure differential (PSID) across theevaporator 2710. The sensors 2710 a and 2710 b and/or components orsubsystems thereof may further be configured and arranged to measure achilled water supply (CWS) temperature (sensor 2710 a) and the chilledwater return (CHR) temperature (sensor 2710 b) associated with theevaporator 2710 (at block 2906).

The demand flow evaporator routine, in turn, calculates a flow rate(GPM) through the evaporator 2710 (at block 2908) based on the detectedand measured pressure and temperature information. In particular, thedemand flow processor accesses the detected values stored in, forexample, the memory as directed by the demand flow control routine. Theevaporator flow rate is be calculated according to the formula:

${{{Meas}.\mspace{14mu}{Evaporator}}\mspace{14mu}{Flow}\mspace{14mu}{Rate}} = {{T\&}B\mspace{14mu}{GPM}*\left( \frac{{{Meas}.\mspace{14mu}{Evaporator}}\mspace{14mu}\Delta\; P\mspace{14mu}({PSID})}{{Design}\mspace{14mu}{Evaporator}\mspace{14mu}\Delta\; P\mspace{14mu}({PSID})} \right)^{.5}}$where the T&B GPM represents the full flow evaporator flow rate. Otherformulas may be used. As previously discussed, the sensors 2710 a and2710 b may be high accuracy pressure sensors such arranged to measurethe supply pressure (via sensor 2710 a) and the return pressure (via2710 b). The difference between the measured supply and return pressuresrepresents the pressure differential or loss across the evaporator 2710.

Upon determination of the measured evaporator flow rate (GPM), thedemand flow evaporator routine 2610 and the demand flow control routine2600 may (as shown at block 2910) calculate the current evaporatoroutput capacity (Tons) according to the formula:

${{Evaporator}\mspace{14mu}{Capacity}\mspace{14mu}({Tons})} = {{{Meas}.\mspace{14mu}{Evap}.\mspace{14mu}{Flow}}\mspace{14mu}{Rate}*\left( \frac{{CWR} - {CWS}}{24} \right)}$

In this way, the demand flow evaporator routine empirically calculatesthe output capacity each evaporator 2710 (e.g., each evaporator part)operating in conjunction with the chilled water plant 2700.

The demand flow control routine and the demand flow evaporator routine,in turn, utilize the results from one or more of the previous steps andcalculations to determine a virtual delta T (see block 2912). Aspreviously discussed, virtual delta T represents an equivalent chilledwater delta T that would result if a constant volume pumping algorithmwere utilized under the current operating conditions. The virtual deltaT may be calculated according to the formula;

${{Virtual}\mspace{14mu}{Delta}\mspace{14mu} T} = {{Design}\mspace{14mu}{Delta}\mspace{14mu} T*{\left( \frac{{Evaporator}\mspace{14mu}{Capacity}\mspace{14mu}({Tons})}{{Design}\mspace{14mu}{Evaporator}\mspace{14mu}{Capacity}\mspace{14mu}({Tons})} \right).}}$

The demand flow control retained and the demand flow evaporator routine,at block 2914, utilize the results and information from one or more ofthe preceding algorithm steps to determine a pressure curve set pointfor the evaporator 2710. The pressure curve setpoint may be determinedaccording to the formula:

${{PC}\mspace{14mu}{Setpoint}} = {{Design}\mspace{14mu}{{Evap}.\mspace{14mu}\Delta}\; P\mspace{14mu}({PSID})*\left( \frac{{Virtual}\mspace{14mu}{Delta}\mspace{14mu} T}{{Baseline}\mspace{14mu}{Delta}\mspace{14mu} T} \right)^{P\mspace{14mu}{Exponent}}}$where the baseline delta T represents the full load delta T chosen orselected at the time of commissioning of the chilled water plant 2700.The baseline delta T may (and often will) correspond to the design deltaT.

In another embodiment, the demand flow control retained and the demandflow evaporator routine, at block 2914, may utilize the results andinformation from one or more of the preceding algorithm steps inconjunction with the measured active pressure differential (PSID) valueto determine the pressure curve set point for the evaporator 2710. Thepressure curve setpoint according to this alternate embodiment may bedetermined according to the formula:

${{PC}\mspace{14mu}{Setpoint}} = {{Active}\mspace{14mu}{{Evap}.\mspace{14mu}\Delta}\; P\mspace{14mu}({PSID})*{\left( \frac{{Measured}\mspace{14mu}{Delta}\mspace{14mu} T}{{Design}\mspace{14mu}{Delta}\mspace{14mu} T} \right)^{P\mspace{14mu}{Exponent}}.}}$

The demand flow evaporator routine and the demand flow control routinemay cooperate to implement a critical zone reset portion of theoperational strategy (see block 2916) in order to adjust the operationof the chilled water plant 2700 to altered demand requirements. Forexample, if cooling demand is lowered, then a critical zone reset mayalter the operating or current delta T linearly towards the design deltaT. In operation, a decrease in demand from the chilled water plant 2700may trigger a critical zone reset that causes the 15-degree currentoperating delta T to shift towards the 16-degree design delta T. Theinputs from the air handling unit are used to control the refrigeranttemperature. Accordingly, the pressure setpoint for one or more pumpsoperating throughout the chilled water plant 2700 may be decreased inorder to decrease the chilled water flow therethrough. The critical zonereset setpoint can be calculated according to the linear formula:Y=M*X+B,where M is the slope of the line as defined by (Y2−Y1)/(X2−X1); X is thecurrent value of the chosen critical zone parameter; and B is theY-intercept value. The Y-intercept value is selected from the minimum ormaximum Y−M*(Min or Max critical zone (CZ) value). The minimum ormaximum critical zone (CZ) values are site-specific parameters selectedor identified at the time of commissioning of the chilled water plant2700 and the demand flow device 2502. For example, in oneimplementation, the humidity within a building or area may be ofimportance to a user, in this instance the critical zone values may beselected based on measured humidity and/or temperature values in thearea of interest. In another embodiment, the sensor 2746 b maymonitoring the temperature and flow into the air handling unit 2746 inorder to determine if and when the supply temperature falls below athreshold or value necessary to provide the desired cooling. In thisembodiment, the operation and performance of the air handling unit 2746may provide the requisite minimum critical zone (CZ) value feedback orcontrol the demand flow device 2502. Other values and parameters may bedetermined based on the requirements of a specific implementation.

The demand flow evaporator routine and the demand flow control routinemay, as indicated at block 2918, determine the temperature at thedecoupler or bypass 128 connecting the primary loop 104 and secondaryloop 108. The detected temperature difference may, in turn, be utilizedto determine the existence of a flow imbalance between the loops 104 and108. The temperature in the de-coupler 128 ranges between predeterminedminimum and maximum temperatures, and the bridge offset may becalculated via a linear equation that adjusts pressure setpoint (asindicated by the reference “C”) associated with the water pump 2720 tobalance the flow between these loops.

The calculated or critical zone reset pressure curve setpoint may, inturn, be communicated from the demand flow control routine and demandflow processor via the communications module to the pump 2716 (see block2920). The change in operating pressure of the pump 2716 to thecalculated or new pressure curve setpoint alters the pressure and flowrate through the evaporator 2710.

Subsequently, the demand flow pump routine may determine (at block 2922)if additional components, pumps, etc. require evaluation and reset. Ifadditional pumps and compressors require evaluation, then the demandflow pump routine calculates a new or optimal pump setpoint for theadditional pump (see block 2924). The demand flow pump routine 2614repeats (at block 2926) the calculations for each identified and/oroperational pump in the chilled water plant 2700.

The calculated pressure curve set point(s) may, in turn, be communicatedfrom the demand flow control routine and demand flow processor via thecommunications module to the remaining pump(s) (see block 2928). Toillustrate with a specific example, an exemplary chilled water plantoptimized and managed in accordance with Demand Flow variable pressurecurve logic is shown in FIG. 13. In the example, the 44 degree chilledwater produced in the primary loop 104 is circulating at a flow rateequal to 899 gallons per minute (GPM) to maintain a 2.6 pressuredifferential (PSID) across the evaporator 2710. Similarly, the secondaryloop 108 circulates the chilled water through the secondary pump 2720 ata flow rate 899 GPM (with the VFD driving the secondary pump at 45 Hz)and a 36 PSID. At this flow rate and pressure differential, the airhandling unit 2746 receives sufficient chilled water flow to cool theoffice space 3100 to a desired temperature. The temperature of thechilled water exiting the air handling unit 2746, in this example,increases from 44 degrees to 60 degrees and circulates from thesecondary loop 108 back to the primary loop 104. In a similar manner,heat from the 60 degree chilled water is transferred from the evaporator2710 to the condenser 2712 via a refrigeration cycle established betweenthe two components. The condenser 2712 with the fan 2730 operate tomaintain a 618 GPM flow rate and a 7 PSID across the condenser 2712. Bybalancing the flow rates between these loops as a function of thepressure setpoints associated with the pumps 2716 and 2720 and fan 2730,the evaporator 2710, the condenser 2712 and the air handling unit 2746may be operated efficiently at flow rates and pressure outside of theiroriginal design parameters. This, in turn, provides additionaloperational flexibility as well as increased efficiency because no onecomponent or element is required to compensate for the inefficientoperation of the remaining components under varying demand conditions.

FIG. 14 shows one embodiment of controlling a pressure set point of thecondenser. In this example method, adjustments of the set point down andup are handled incrementally, such as at 1.0 for down and 0.5 for up,but other adjustments may be used. The setpoint for temperature islimited between 85 and 65, but other ranges may be used. The temperaturerange is mapped linearly for loads from 100% to 65%.

In act 1401, the method starts. The total power is read in act 1402. Thetotal power is a sum of the condenser fan, chiller (e.g., compressor),and refrigerant pump powers, but other power measures may be used. Inact 1403, the chilled water flow is read. The value of the old orprevious flow is compared to the newly read flow in act 1404. If equal,then an alarm is generated in act 1405 and a new total power is read inact 1406. If the new power is less than the old power in act 1407, thenthe condenser pressure set point is changed in act 1408 in the samedirection as the last change in set point. If the new power is more thanthe old power in act 1409, then the condenser pressure set point ischanged in act 1410 in the reverse direction to the last change in setpoint.

If the old flow and new flow from act 1404 are equal, the flow isverified to be within a site specific range (e.g., 500-4,000) in act1411. If within the range, then the tons of cooling is calculated in act1412 using chilled water delta T from the chilled water loop temperaturesensors and the chilled water flow meter in act 1413. If not within therange, then acts 1405-1408 are performed.

In addition to reading in acts 1402 and 1403, the method waits 10minutes or other amount of time in act 1414. After waiting 10 minutes inact 1414, the pressure set point for the condenser is read in act 1415.An average evaporator refrigerant offset (ERO) (see FIG. 19) issubtracted from the pressure set point in act 1416. If the fan speed ofthe condenser is greater than 95.5%, the set point of the condenserpressure is raised by 0.5 degrees in act 1417, and the process returnsto act 1401. If the fan speed is less than or equal to 99.5%, thecurrently calculated tons from act 1412 is compared to previous tons inacts 1418 and 1419. The percent change in tons is calculated. If the newton value is less than the old ton in act 1418, then a new total poweris read in act 1420. If the new power is less than the old power in act1421, the percent change in tons is compared to percent change in powerin act 1422. If the percent change in power is less than the percentchange in tons, the condenser pressure set point is changed in the samedirection as the previous change in set point in act 1423. If thepercent change in power is more than the percent change in tons, thecondenser set point is changed in a reverse direction as the previouschange in set point in act 1424. If the new power is greater than theold power in act 1425, then act 1424 is performed.

If the new ton value is greater than the old ton in act 1419, then a newtotal power is read in act 1426. If the new power is less than the oldpower in act 1427, then the condenser pressure set point is changed inthe same direction as the previous change in set point in act 1428. Ifthe new power is greater than the old power in act 1429, then thepercent change in tons is compared to percent change in power in act1430. If the percent change in power is less than the percent change intons, the condenser pressure set point is changed in the same directionas the previous change in set point in act 1428. If the percent changein power is more than the percent change in tons, the condenser setpoint is changed in the reverse direction as the previous change in setpoint in act 1431.

FIG. 14 also includes an example exception handling. In act 1440, theaverage power load of the chillers is greater than the chiller load upswitch point for more than 30 seconds or other period, then thecondenser fan speed is compared to a threshold, such as 99.5%, in act1441. The load up switch point is the calculated value of compressorpower that would require turning on an additional chiller to meet thecooling load. The load up switch point is calculated by dividing theactual compressor power compared to the maximum compressor power. Atlower condenser temperatures, the switch up set point is higher (e.g.,75° F. and 99.5%), and at higher condenser temperatures, the switch upset point is lower (e.g., 85° F. and 80%). If the fan speed is less,then the condenser pressure set point is lowered in act 1445, such as by2 degrees so long as the not being below a value, such as 50 degrees. Inact 1442, the adjustment of the set point upwards is prevented for aperiod, such as two times the 10 minute period of act 1414. The setpoint control loop of acts 1401-1431 is then performed in act 1443. Ifthe fan speed is greater than the threshold in act 1441, the number ofcondenser fans being used is read in act 1444. If the number of usedfans is less than the maximum number, then act 1445 is performed. If thenumber is equal in act 1444, the act 1443 is performed.

FIG. 15 shows one embodiment of determining air handling unit demand.The zone calculations are performed by the air handling unit controller,and the zone demand calculations are performed by a pump controller.

In act 1501, the position of the value is read. The air supplytemperature is read in act 1502. If the air supply temperature isoutside a range (e.g., 0-150 degrees) as compared in act 1503, then analarm is generated in act 1504. In either case, a temperature correctionfactor is set to 0 if the air handler unit is off or the value is inhand (i.e., overridden by the operator to the On position) in act 1550.If not, the supply air temperature is checked for being within athreshold amount (e.g., 1 degree) of the air temperature set point inact 1505. If within, then the temperature correction factor is set to1.0. If not, then the temperature correction factor is calculated as1+((supply air temperature−supply air temperature set point)− 1/10) whenthe supply air temperature is greater than the set point and as1+((supply air temperature−supply air temperature set point)+ 1/10) ifthe supply air temperature is less than the set point in act 1506.

In act 1507, the critical zone parameter is calculated as 1+(zonerating/10). In act 1508, the air handling unit valve is checked forproper function. If not working, then the temperature correction factoris set to 0.0 in act 1509. In act 1510, the valve size correction iscalculated as valve size/average valve size for the air handling unitssharing chilled water.

In act 1511, the zone demand is calculated as the valve position×thezone parameter×valve size correction×temperature correction factor×unitstatus. In act 1512, the zone demand calculation is performed for all ofthe other valves on the same pumping system (e.g., air handling unitssharing chilled water). In act 1513, the weighted average is calculatedas (average zone demand+maximum of the zone demands)/2.

In act 1508, the proper function is checked. FIG. 16 shows oneembodiment for air handling unit valve fault detection. The valve faultdetection is performed by the air handling unit controller, and the zonedemand calculation is performed by the pump controller.

In act 1601, the discharge air temperature at the air handling unit isread periodically (e.g., every 60 seconds) if the fan of the airhandling unit being on for a period, such as 10 minutes. In act 1602,the supply air temperature is checked to be within a range, such as0-150 degrees. If outside the range, an alarm is generated in act 1603as the sensor may not be working. An alarm indicating that the airhandling unit chilled water valve is not working or tuned is also issuedin act 1604, and the temperature correction factor used in the processorof FIG. 15 is changed to 0.0 in act 1605.

If the supply air temperature is within the range in act 1602, then theperiodic error is calculated as the discharge air temperature−dischargeair set point in act 1606. In act 1607, the cumulative error iscalculated as the error from act 1606 plus the previous cumulativeerror. This calculation is performed periodically, such as every 60seconds in synchronization with the reading of the discharge airtemperature in act 1601. In act 1607, the process continues back to thestart of the measurement process in act 1601.

Every hour or other period, an average error is calculated in act 1608(e.g., cumulative error/60). If the average error is within a range,such as +/−2 degrees, the valve and control loop are operating properlyas determined in act 1609. The cumulative error is reset to 0.0 in act1610 and the process continues back to the start of the measurementprocess in act 1611. If the average error is outside the range, then thevalve or loop tuning are not operating properly as determined in act1612. Acts 1604, 1605, 1611, and 1612 are then performed.

FIG. 17 shows one embodiment of controlling the flow of chilled water.The primary pump of the chilled water is controlled by a controller,such as a pump controller and/or other controller.

In act 1701, the building weighted averages from the air handling units(see act 1513 of FIG. 15) are read. If the weighted average is above 0and has not changed over a time period or since the last performance ofthe act, or if all the weighted averages are 0, then the buildingautomation network has failed. Act 1702 shows performing this test. Iffailure occurs, an alarm is issued in act 1703 and the primary pumpspeed is set to provide a constant delta T at the header in the waterplant in act 1704. If the test of act 1702 passes, a weighted average iscalculated in act 1705 as (the average building demand+maximum buildingdemand)/2. In act 1706, the weighted average is checked to determine ifthe average is within a range, such as 85-95. If within the range, thepressure differential set point (DP) between chilled water supply andreturn line pressures is not changed in act 1707. If outside the rangeby being below in act 1708, the DP set point is adjusted down, such asby 1 PSID, in act 1709. If outside the range by being above in act 1710,the DP set point is adjusted up, such as by 1 PSID, in act 1711. Theadjustment of acts 1709 and 1711 occur periodically, such as every 10minutes.

FIG. 17 also shows control of the chilled isolation valve, such as whenthe primary pump is headered. In act 1720, the chiller % electrical loadon the actual power load divided by the maximum power load operatingchillers is read. In act 1721, the average % electrical load of theoperating machines is calculated once the machines have been running fora period, such as 10 minutes. The chiller isolation valve is controlledin act 1722 in 1% output steps at intervals, such as 5 minute intervals,to get within a range (e.g., +/−5%) of the average % load. If the lowestor higher % load value is more than 20% from the average, an alarm isissued in act 1723.

FIG. 18 shows one embodiment of control of the chiller load. For achilled water plant with multiple air cooled chillers, the number ofchillers operating at a given time varies based on the load.

In act 1801, the % of the chiller current limit (the maximum allowableoperating limit on the machine) is read from the machine. In act 1802,the % of chiller load is calculated by dividing the actual power by themaximum allowable power. In act 1803, the chilled water supplyTemperature is read. In act 1804, the leaving condenser refrigeranttemperature is read.

In act 1805, the % of the used load is calculated as the % load×currentlimit. The calculation is performed for each chiller. In act 1806, theaverage used load is calculated as the sum of available tonnage for alloperating chillers divided by the number of operating chillers. In act1807, the load down switch point is calculated. The condenserrefrigerant temperature set point is varied from one temperature (e.g.,75) to another (e.g., 85), and the load down switch point is varied from60% to 50%. In act 1808, the average used load is compared to the loaddown switch point. If the average used load is less than the load downswitch point for a period (e.g., 20 minutes), a chiller is removed or nolonger operated.

In act 1809, a time check is performed. If not during the day time(e.g., 6 am to 3 pm), the average used load is compared to the load upswitch point in act 1810. If the average used load is greater than theload up switch point for a period (e.g., 40 minutes), a chiller is addedor started operating. In act 1811, the chilled water set point islowered by an amount (e.g., 2 degrees) unless at a bottom limit (e.g.,50 degrees) if the average load is greater than the chiller load upswitch point.

If during the day, the chilled water supply temperature is greater thanthe set point plus 2 degrees for a period (e.g., 20 minutes), a chilleris added in act 1812. If chilled water supply temperature is greaterthan the set point plus 5 degrees for a different period (e.g., 5minutes), a chiller is added. This +5 degree check is not performedwithin 10 minutes of having added a chiller.

In act 1813, a chiller is added if the average used load is greater thanthe load up switch point for a period (e.g., 20 minutes). In act 1814,the chilled water set point is lowered by an amount (e.g., 2 degrees)unless at a bottom limit (e.g., 50 degrees) if the average load isgreater than the chiller load up switch point.

FIG. 19 shows one embodiment for resetting the chilled watertemperature. In act 1901, the temperature of the refrigerant leaving thecondenser is read. If the temperature is outside a range (e.g., 0-150degrees) as determined in act 1902, an alarm is issued in act 1903 andthe chilled water temperature is reset to a maximum value in act 1904.

If within the range, the chilled water pump operation is checked in act1905. In act 1906, if the pump is operating at more than 50%, theleaving condenser temperature is compared to a release set point and thechiller load is compared to a threshold (e.g., 90%). If the temperatureis less than the set point and the load is less than the threshold, thenthe chilled water temperature is compared to a threshold in act 1907. Ifgreater than the threshold, then the chilled water temperature is resetdown by an amount (e.g., 0.5 degrees) in act 1908, the ERO is added inact 1909, and the chilled water set point is limited within a range inact 1910. After waiting a period (e.g., 15 minutes) in act 1911, theprocess continues back to start in act 1912 in act 1901. If the chilledwater is less than the threshold in act 1907, the process proceeds toact 1911.

If the criteria for act 1906 are not met, then the chilled water iscompared to a threshold (e.g., 42 degrees) in act 1912. If less than thethreshold, then the chilled water temperature is reset up by an amount(e.g., 0.5 degrees) in act 1913, the evaporator refrigerant offset (ERO)is added in act 1914, and the chilled water set point is limited withina range in act 1915. After waiting a period (e.g., 15 minutes) in act1916, the process continues back to start in act 1901. If the chilledwater is less than the threshold in act 1912, the process proceeds toact 1916.

If the check of act 1906 shows the chilled water pump operating lessthan the threshold capacity (e.g., 50%), the chilled water is comparedto a threshold (e.g., 48 degrees) in act 1917. This threshold may bedifferent (e.g., 42 degrees) for environments with humidity control(e.g., hospital). If less than the threshold, then the chilled watertemperature is reset up by an amount (e.g., 0.5 degrees) in act 1918,the ERO is added in act 1919, and the chilled water set point is limitedwithin a range in act 1920. After waiting a period (e.g., 15 minutes) inact 1921, the process continues back to start at 1901 in act 1912. Ifthe chilled water is less than the threshold in act 1917, the processproceeds to act 1920.

FIG. 19 also shows calculation of the ERO. In act 1940, the evaporatorapproach is calculated as the evaporator leaving water temperature−theevaporator refrigerant temperature. For new chillers, the ERO is set tothe greater of 0.0 or (approach temperature −0.5 degrees) in act 1941.For older machines, the ERO is set to the greater of 0.0 or (approachtemperature −2.0 degrees).

Another check is shown in act 1960. An alarm is sent if the chillercharge is above 50. The chiller charge is calculated as (evaporatorapproach²×100)/((condenser approach+evaporator approach)×2).

Other processes, different processes, or fewer processes may beprovided. Additional, different, or fewer checks may be provided. Otherthresholds, amounts, temperatures, times, periods, increments, changes,or adjustments may be used.

While various embodiments of the invention have been described, it willbe apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible that are within the scopeof this invention. In addition, the various features, elements, andembodiments described herein may be claimed or combined in anycombination or arrangement.

What is claimed is:
 1. A method for controlling an air cooled chiller,the method comprising: condensing refrigerant in a condenser at varyingrates and temperatures based on load conditions and total power use;evaporating the refrigerant in an evaporator at varying rates andtemperatures based on the load conditions; controlling, in part,pressure of the refrigerant with a compressor; sensing pressure betweencondenser and evaporator; pumping the refrigerant with a refrigerantpump in response to pressure in between the condenser and theevaporator; and storing, in a receiver, the refrigerant received fromthe refrigerant pump and provided to the evaporator.
 2. The method ofclaim 1 wherein controlling the pressure comprises controlling, in part,the pressure with an expansion valve.
 3. The method of claim 1 whereinpumping comprises pumping in response to the pressure being over athreshold.
 4. The method of claim 1 wherein pumping comprises bypassingthe refrigerant pump.
 5. The method of claim 1 further comprising:measuring power used by the air cooled chiller; adjusting a fan speed ofair flow fans of the condenser as a function of the power.
 6. The methodof claim 1 further comprising: sensing a position of a valve at an airhandling unit fluidly connected with the air cooled chiller; andcontrolling a refrigerant temperature as a function of the position.